netdev - Re: [PATCH v3 bpf-next 5/7] libbpf: move bpf_{helpers,endian,tracing}.h into libbpf

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Message-ID: <4fcbe7bf-201a-727a-a6f1-2088aea82a33@gmail.com>
Date:   Fri, 4 Oct 2019 09:44:44 -0600
From:   David Ahern <dsahern@...il.com>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v3 bpf-next 5/7] libbpf: move
 bpf_{helpers,endian,tracing}.h into libbpf

On 10/4/19 9:27 AM, Andrii Nakryiko wrote:
> On Fri, Oct 4, 2019 at 7:47 AM David Ahern <dsahern@...il.com> wrote:
>>
>> On 10/3/19 3:28 PM, Andrii Nakryiko wrote:
>>> Move bpf_helpers.h, bpf_tracing.h, and bpf_endian.h into libbpf. Ensure
>>> they are installed along the other libbpf headers. Also, adjust
>>> selftests and samples include path to include libbpf now.
>>
>> There are side effects to bringing bpf_helpers.h into libbpf if this
>> gets propagated to the github sync.
>>
>> bpf_helpers.h references BPF_FUNC_* which are defined in the
>> uapi/linux/bpf.h header. That is a kernel version dependent api file
>> which means attempts to use newer libbpf with older kernel headers is
>> going to throw errors when compiling bpf programs -- bpf_helpers.h will
>> contain undefined BPF_FUNC references.
> 
> That's true, but I'm wondering if maintaining a copy of that enum in
> bpf_helpers.h itself is a good answer here?
> 
> bpf_helpers.h will be most probably used with BPF CO-RE and
> auto-generated vmlinux.h with all the enums and types. In that case,
> you'll probably want to use vmlinux.h for one of the latest kernels
> anyways.

I'm not following you; my interpretation of your comment seems like you
are making huge assumptions.

I build bpf programs for specific kernel versions using the devel
packages for the specific kernel of interest.

> 
> Nevertheless, it is a problem and thanks for bringing it up! I'd say
> for now we should still go ahead with this move and try to solve with
> issue once bpf_helpers.h is in libbpf. If bpf_helpers.h doesn't work
> for someone, it's no worse than it is today when users don't have
> bpf_helpers.h at all.
> 

If this syncs to the github libbpf, it will be worse than today in the
sense of compile failures if someone's header file ordering picks
libbpf's bpf_helpers.h over whatever they are using today.